Allow using wxWindowUpdateLocker conditionally

This can be useful if the window needs to be frozen only if some
run-time condition holds true.

include/wx/wupdlock.h

@@ -19,12 +19,29 @@
 class wxWindowUpdateLocker
 {
 public:
+    // Prefer using the ctor below if possible, this ctor is only useful if
+    // Lock() must be called only conditionally.
+    wxWindowUpdateLocker() : m_win(NULL) { }
+
     // create an object preventing updates of the given window (which must have
     // a lifetime at least as great as ours)
     explicit wxWindowUpdateLocker(wxWindow *win) : m_win(win) { win->Freeze(); }
 
+    // May be called only for the object constructed using the default ctor.
+    void Lock(wxWindow *win)
+    {
+        wxASSERT( !m_win );
+
+        m_win = win;
+        win->Freeze();
+    }
+
     // dtor thaws the window to permit updates again
-    ~wxWindowUpdateLocker() { m_win->Thaw(); }
+    ~wxWindowUpdateLocker()
+    {
+        if ( m_win )
+            m_win->Thaw();
+    }
 
 private:
     wxWindow *m_win;